Key components
In Open RAN, the RAN is split into three main blocks: the Radio Unit (RU), the Distributed Unit (DU), and the Centralised Unit (CU). The RU performs basic RF and low-PHY functions such as precoding and beamforming. The DU performs additional PHY functions such as scrambling and modulation, and handles MAC and RLC functions such as radio resource allocation. The CU handles functions such as SDAP and RRC.
RAN Intelligent Controller (RIC) is an optional virtualized optimization technology for 5G. It adds programmability to a new or existing RAN and allows SON-like (Self-Optimizing Networks) capabilities. The optimization in RIC is boosted by policy-driven closed loop automation and AI/ML. In O-RAN deployments, there are the near real-time RIC (near-RT RIC) which operates on sub-100 millisecond intervals, and the non real-time RIC (non-RT RIC) which operates on intervals longer than 1 second. The near-RT RIC houses xApps, which are programs that optimize functions such as beamforming, traffic steering, and power control. The non-RT RIC houses rApps, which manage longer-term policies such as network slicing.
The fronthaul is an eCPRI-based interface with various splits, the most popular being 7.2x. It connects radio units with distributed units of different vendors. It is a focus area in Open RAN.
Near-RT RIC
The near-RT RIC houses xApps, which require numerous components to support their adequate and safe functioning:
- Internal messaging infrastructure connects the xApps, platform services, and interface (O1, A1, E2) terminations to each other. O-RAN Alliance specifies not what technology the near-RT RIC must use, but there are certain functionalities that this internal messaging infrastructure must provide.
- The subscription manager allows xApps to listen to functions exposed over the E2 interface.
- The security sub-system prevents malicious xApps from leaking sensitive RAN data or negatively affecting RAN performance.
- The RAN Network Information Base (NIB) Database stores information from the E2 interface, typically KPMs from RANs and UEs. The UE-NIB contains information about UEs, specifically their identity key (UE-ID), which allows for xApps to control UEs. These NIBs may be queried through the Shared Data Layer API, usually by xApps.
- Conflict mitigation prevents RAN performance from being degraded by xApps making conflicting managing decisions on RANs. This is an emerging functionality and is not included in all near-RT RIC implementations.
- xApps are typically realized as Kubernetes or Docker containers. The O-RAN alliance only stipulates that a few KPIs are exposed to the xApps.
Open Source Near-RT RICs
Popular open source implementations of Near-RT RICs include FlexRIC and ORAN SC RIC. Research is continuously being done on facilitating compliance with Open RAN standards and interoperability between RICs and RANs of different vendors.
Non-RT RIC
The non-RT RIC houses rApps, which handle functions of RANs on a timescale greater than 1 second. The R1 termination connects rApps to the non-RT RIC.